## **Core Concept**
Thromboembolism associated with oral contraceptive pills (OCPs) is primarily related to the **estrogen** component. Estrogen can increase the synthesis of clotting factors in the liver, leading to a hypercoagulable state.

## **Why the Correct Answer is Right**
The estrogen component of OCPs is known to cause several changes in coagulation factors, including increases in factors II, VII, VIII, IX, X, and XII. This results in an increased risk of venous thromboembolism (VTE), which includes both deep vein thrombosis (DVT) and pulmonary embolism (PE). The risk is particularly notable in the first year of use and in women who are over 35 years old and smoke.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while progestins can have some effects on coagulation, they are not primarily associated with the increased risk of thromboembolism to the same extent as estrogen.
- **Option B:** This option might seem plausible due to the role of hormones in OCPs, but it's not specified enough to be correct, and typically, the estrogen component is singled out for its effect on clotting factors.
- **Option C:** This option is incorrect because the inactive ingredients or fillers in OCPs do not have a pharmacological effect on coagulation or thromboembolism risk.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the risk of thromboembolism is higher in women using OCPs who are over 35 years old and smoke. This population is particularly advised against the use of combined hormonal contraceptives due to this increased risk.

## **Correct Answer:** . Estrogen